1. Field of the Invention
The present invention relates to toner for developing an electrostatic charge image which has excellent characteristics including chargeability and is suitable for use in the image formation in such application as electrophotography, a method for efficiently producing the toner, the toner which is produced by the method, and a developing agent for an electrostatic image produced by using the toner and a method for forming an image by using the toner.
2. Description of the Related Art
A method in which image information is visualized via an electrostatic image as in electrophotography is widely used currently in various fields. The general electrophotography method consists of the steps of forming an electrostatic image on a photorecepter after charging and exposure, developing the electrostatic image by use of a developer containing toner particles, and visualizing the developed image via transfer and fixation.
As is generally known, there are two types of the developer, that is, a two-component developer which comprises toner particles and carrier particles, and a one-component developer which comprises either magnetic toner particles or non-magnetic toner particles. The toner particles of these developers are usually prepared by a blending/pulverizing process. The blending/pulverizing process comprises the steps of melt-blending a thermoplastic resin or the like with a pigment, a charge controller and a release agent such as a wax, pulverizing the resulting product after cooling, and sieving the pulverized product to obtain desired toner particles. If necessary, for the purpose of improving the properties such as fluidity and cleanability of the thus prepared toner particles, inorganic and/or organic particles are added to the surface to the toner fine particles.
Usually, the shapes of the toner particles prepared in the above-mentioned blending/pulverizing process are irregular and the surface compositions of the toner particles are not uniform. The shapes and surface compositions of the toner particles vary subtly depending on the pulverizability of the materials and the pulverizing conditions. However, it is difficult to control intentionally the shapes and surface compositions of the toner particles within a desired range. Furthermore, if the materials of the toner particles are particularly easy to pulverize, it often occurs that the toner particles are further finely pulverized in a developing apparatus by a mechanical force such as a shearing force and that the shapes of the toner particles change. Accordingly, a problem to be encountered in the case of the two-component developer is prompted deterioration of the chargeability of the developer due to tenacious adhesion of the fine toner particles to the carrier surface, while problems to be encountered in the case of the one-component developer are, for example, the broadening of the particle size distribution accompanied by the dissipation of the fine toner particles, and the decline in quality of image as a result of the deterioration in developing performance of the toner due to the variation of the shapes of the toner.
Further, if the shapes of the toner particles are irregular, a sufficient fluidity cannot be obtained even if a fluidity aid is added. The fluidity decreases with the passage of time because a mechanical force such as a shearing force causes the fluidity aid particles to move to dents in the toner particles to be embedded therein. Consequently, the qualities such as developability, transferability and cleanability become worse. In addition, if such toner particles are recovered by a cleaning treatment, restored to the developing apparatus, and recycled, the quality of the obtained image tends to be inferior. If the amount of the fluidity aid is increased in order to prevent the above-mentioned problems, new problems will be, for example, the generation of black spots at the photorecepter and the dissipation of the particles of the fluidity aid.
Meanwhile, if the toner particles contain a release agent such as a wax, the release agent is exposed on the surface of the toner particles according to the combination of the release agent and a thermoplastic resin. In particular, if the toner particles consist of a resin, whose elasticity is arised by adding a polymer component and which is somewhat difficult to be pulverized, and a fragile wax such as polyethylene, a significant proportion of the polyethylene is exposed on the surface of the toner particles. Although these toner particles are advantageous in terms of release in the fixing process and removing the untransferred toner from the photorecepter, a mechanical force such as a shearing force inside the developing apparatus causes the polyethylene to separate from the toner particles and to migrate easily to such members as developing rolls, a photorecepter and carriers. Consequently, the contamination of these members lowers the reliability of the developer.
Because of this background, recently, an emulsion polymerization/flocculation process has been proposed as a method for producing toner particles whose shapes and surface compositions are intentionally controlled. The emulsion polymerization/flocculation process comprises the steps of preparing a resin dispersion liquid by an emulsion polymerization on the one hand, preparing a colorant dispersion liquid comprising a solvent and a colorant dispersed therein on the other hand, blending the two dispersion liquids to prepare flocculated particles having a particle size corresponding to the toner particle diameter, and then heating the blend to fuse the resin and the colorant to obtain toner particles. According to the emulsion polymerization/flocculation process, it is possible to control the shapes of the toner particles at will from an irregular shape to a sphere by selecting the heating temperatures.
In the emulsion polymerization/flocculation process, however, it is difficult to control intentionally the structure and the composition of the surface of the toner particles, because the composition in the region ranging from toner particle interior to the particle surface is made uniform by the fusion of the flocculated particles in a uniformly blended state. If the flocculated particles contain a release agent, the release agent is localized on the surface of the toner after fusion, which may lead to a filming phenomenon and the embedding of an external additive for improving fluidity into the toner particle interior.
In an electrophotographic process, in order to maintain and exhibit the quality of toner in a stable manner, it is necessary to inhibit the exposure of the release agent on the surface of toner particles, to increase the surface hardness of the toner particles and to increase the surface eveness of toner particles. Despite of the possible problems ascribable to the release agent exposed on the surface of toner particles, from the viewpoint of the toner quality at fixing process, it is desirable that the release agent be localized in the vicinity of the surface of toner particles.
Recently, because of a rise in demand for a high-quality image, especially for a high-quality color image, the diameter of the toner particles is remarkably reduced in order to perform a high-precision image. However, even if the particle sizes of conventional toner, whose particle distribution is too broad, are simply reduced, it is difficult to achieve a high-quality image and a high reliability simultaneously, because serious problems such as contamination of developing rolls, electrically charging rolls, electrically charging blades, photorecepter, carriers, and the like as well as dissipation of toner particles are caused by the toner particles having diameter in shorter regions of the particle size distribution. Further, if the toner particles having such a broad particle distribution are used in a system comprising means for cleaning, for recycling the toner, and the like, the reliability of the system is poor. In order to achieve a high-quality image and a high reliability simultaneously, it is necessary to narrow the width of the particle size distribution and to reduce the particle sizes.